The present invention relates to methods and compositions for treating subterranean formations, and more specifically, to improved methods and compositions for reducing the viscosity of viscosified treatment fluids.
Viscosified treatment fluids are used in a variety of operations in subterranean formations. For example, viscosified treatment fluids have been used as drilling fluids, fracturing fluids, and gravel packing fluids. Viscosified treatment fluids generally have a viscosity that is sufficiently high to suspend particulates for a desired period of time, to transfer hydraulic pressure, and/or to prevent undesired leak-off of fluids into the formation.
Most viscosified treatment fluids include gelling agent molecules that are crosslinked to increase their viscosity. The gelling agents typically used in viscosified treatment fluids are usually biopolymers or synthetic polymers. Common gelling agents include, inter alia, galactomannan gums, cellulosic polymers, and polysaccharides. The crosslinking between gelling agent molecules occurs through the action of a crosslinker. Conventional crosslinkers generally comprise boron, aluminum, antimony, zirconium, magnesium, or titanium.
In some applications e.g., in subterranean well operations, after a viscosified treatment fluid has performed its desired function, the fluid may be “broken,” meaning that its viscosity is reduced. Breaking a viscosified treatment fluid may make it easier to remove the viscosified treatment fluid from the subterranean formation, a step that generally is completed before the well is returned to production. The breaking of viscosified treatment fluids is usually accomplished by incorporating “breakers” into the viscosified treatment fluids. Traditional breakers include, inter alia, enzymes, oxidizers, and acids. As an alternative to using traditional breakers, a viscosified treatment fluid may break naturally if given enough time and/or exposure to a sufficient temperature. This may be problematic, however, as it may increase the amount of time before the well may be returned to production.
In some situations, the use of traditional breakers is associated with premature and/or incomplete viscosity reduction. This may be problematic. For example, in a fracturing operation, a viscosified treatment fluid may be introduced into a subterranean formation at a pressure sufficient to create or enhance at least one fracture therein. Premature viscosity reduction can decrease the quantity and/or length of fractures generated within the formation, and therefore may decrease the likelihood that the fracturing operation will result in enhanced production. In addition, premature viscosity reduction can cause particulates like proppants to settle out of the fluid in an undesirable location and/or at an undesirable time. Traditional breakers also can be problematic in that they may chemically degrade gelling agents. As a result, pieces of the degraded gelling agent may adhere to the formation, clogging the pore throats of the formation, and thereby potentially impacting the production of desirable fluids. Moreover, the degradation of gelling agents prevents them from being reused.